JP2000329752A - Method and apparatus for detecting stepped flaw of welded pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately detect the stepped flaw generated in the welded part of a welded pipe regardless of the shape thereof and the height of a stepped part. SOLUTION: A arcuate ultrasonic probe 3 is arranged in opposed relation to the welded part 2 continuously formed to a welded pipe 1 in the longitudinal direction thereof and the welded part 2 is irradiated with ultrasonic waves emitted form the ultrasonic probe 3 so as to be converged to the center angle range thereof and the reflected waves from the welded part 2 are received by the ultrasonic probe 3 and the presence of a stepped flaw is judged on the basis of the received result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a step-like defect occurring in a welded portion of a welded pipe, and an apparatus for detecting this defect in a welded pipe in a pipe manufacturing line.

[0002]

2. Description of the Related Art A welded pipe such as an electric resistance welded pipe transports a strip-shaped metal plate as a raw material in a longitudinal direction, passes through a group of forming rolls arranged in the middle of a transport path, and forms a cylindrical shape from both sides in a width direction. After bending and forming the both ends in the width direction butted by this forming, butch welding, remove the excess portion generated inside and outside of the welded part by cutting, pass through a finishing roll such as Sizer roll and trim the outer shape, It is manufactured by a procedure of cutting to a desired size.

[0003] In the welded pipe thus manufactured,
In the process of bending forming, for example, when a defect such as poor adjustment of the forming roll group occurs, the abutting of both ends of the band-shaped metal plate is not performed correctly, and welding is performed in a state where a step-like positional displacement occurs in the radial direction. There is a risk that this displacement will remain as a defect in the welded part of the product pipe obtained through removal of the excess portion, shape modification, and cutting, resulting in a product pipe with insufficient welded part strength. is there.

In recent years, the application range of the welded pipe manufactured as described above has tended to be expanded to applications requiring high strength, such as a high-pressure fluid transfer pipe. In the manufacture of pipes, it is important to detect the step-like defects that occur in the weld as described above and feed back this detection result to the forming roll group for bending forming to reduce the incidence of defective product pipes. Is an important issue.

[0005] The detection of the step-like defects as described above is conventionally performed by ultrasonic flaw detection used for detecting various kinds of defects. FIG. 12 and FIG. 13 are explanatory diagrams of a conventional step-like defect detection method. In the drawing, reference numeral 1 denotes a welded pipe, and as shown in the figure, a welded portion 2 which has been welded by abutment with each other in a state where a stepped displacement has occurred in the radial direction.
have.

[0006] When such a welded pipe 1 is to be inspected, a surface wave method utilizing the feature that ultrasonic waves incident at a predetermined angle on the outer surface of the pipe propagate in the circumferential direction on the outer surface. Is widely used. For example, if the target is a carbon steel pipe immersed in water as a propagation medium for ultrasonic waves, the surface waves are formed by injecting ultrasonic waves at an angle of 25 ° to 30 ° with respect to the outer surface of the pipe. Can be generated.

The surface waves generated in this manner propagate along the outer surface of the welded pipe 1 and reach the welded portion 2 as shown by solid arrows in FIG. When a defect is present, it is reflected by the inner surface of the step on the outer peripheral surface side, and propagates in the opposite direction as a surface wave along the outer surface of the welded pipe 1 as shown by a broken arrow in FIG. The presence or absence of a step-like defect in the welded portion 2 can be detected based on the result of receiving the reflected echo at an appropriate position on the outer surface of the tube 1.

[0008]

However, when the surface wave method as described above is employed, as shown in FIG. 13, when a step at an obtuse angle with respect to the outer surface of the welded pipe 1 is generated, the step The traveling direction of the reflected wave is directed inward, as indicated by the dashed arrow in the figure, so that the reception strength of the reflected echo on the outer surface decreases, and the reliability of defect detection based on the reception result decreases. There was a problem.

Further, there is a problem that the detection accuracy of the step-like defect by the surface wave method is affected by the height of the step in the welded portion 2. FIG. 14 is a diagram showing the results of performing defect detection by the surface wave method on welded pipes having stepped defects of various heights, and the horizontal axis represents the step height (mm).
The vertical axis indicates the S / N ratio (dB) of the received reflected echo. As shown in this figure, when using the surface wave method, 0.
For a step having a height of 1 mm or less, the reception result of the reflected echo often has an S / N ratio of 12 dB or less, and the presence or absence of a step-like defect is correctly determined based on such a reception result. It is difficult.

[0010] The height of the welded portion of the welded pipe is 0.1 mm or less with the expansion of the application range as described above.
Increasingly, even minute step-like defects that are difficult to detect by the surface wave method are problematic. In such a case,
Since the defect detection by the surface wave method is difficult, a visual inspection of the product tube is required, and this inspection requires a high degree of skill, and it is possible to avoid the occurrence of individual differences among inspection workers in the detection accuracy. There is no problem,
Since this inspection is performed off-line, there is a problem that a large number of defective products are generated before the inspection result is fed back to the pipe production line.

The present invention has been made in view of such circumstances, and enables a step-like defect occurring in a welded portion of a welded pipe to be accurately detected irrespective of the form and the level of the step. It is an object of the present invention to provide a method for detecting a defect and a method for performing the method in a pipe manufacturing line for a welded pipe.

[0012]

A method for detecting a stepped defect in a welded pipe according to the present invention comprises bending a strip-shaped metal plate into a cylindrical shape from both sides in the width direction and abutting the edges in the width direction. In a method for detecting a step-like defect caused by the edge being displaced in a step-like manner at the time of the abutting welding, the ultrasonic probe including an arc-shaped oscillating portion and a receiving portion is provided. , The central angle, or opposed to the outside of the welded pipe so that the welded portion is included in the range of the diagonal of the central angle,
The reflected wave of the ultrasonic wave emitted from the oscillating portion from the welding portion is captured by the receiving portion, and the presence or absence of the step-like defect is determined based on the reception result.

Further, the stepped defect detecting device for a welded pipe according to the present invention is a welded portion of a welded pipe formed by bending a strip-shaped metal plate into a cylindrical shape from both sides in the width direction and abutting the edges in the width direction. In an apparatus for detecting a step-like defect caused by the edge being displaced in a step-like manner during the abutting welding, an ultrasonic probe including an arc-shaped oscillating section and a receiving section; Positioning means for positioning the stylus on the outside of the welded pipe so that the welded portion is included in the range of the central angle or the diagonal of the central angle, and the welding of ultrasonic waves emitted from the oscillating portion Determining means for determining the presence or absence of the step-like defect based on the result of receiving the reflected wave from the unit to the receiving unit.

According to the present invention, an ultrasonic probe having an arc-shaped oscillating portion and a receiving portion is placed outside a target welded pipe so that the welded portion of the welded pipe has a central angle range of the arc-shaped. Inside, or arranged opposite to be located within the diagonal range of the central angle, and after being emitted from the oscillating unit, ultrasonic waves converging within the central angle range, or the diagonal after this convergence Applying ultrasonic waves diffusing within the range of the welded pipe to the outer surface of the welded pipe, when there is a step-like defect in the welded portion, a reflected wave that is reflected by this step and returns to within the range of the central angle or the diagonal angle The data is received by the receiving unit, and the presence or absence of the defect is determined based on the reception result.

Further, in the apparatus for detecting a stepped defect in a welded pipe according to the present invention, the positioning means may include a step of bending the band-shaped metal plate into a cylindrical shape from both sides in the width direction, and a step of bending the formed band-shaped metal plate in the width direction. Means for detecting a welded portion of a welded pipe moving on a pipe-making line including a step of performing abutment welding of an edge of the probe, and position control for changing a position of the ultrasonic probe based on a detection result of the means. Means.

According to the present invention, the position change of the welded portion caused by the movement on the pipe production line is detected, and the operation of the position control means based on the detection result causes the superposition of the superpositioned outside of the welded pipe. The ultrasonic probe is position-controlled so as to maintain a predetermined positional relationship with respect to the welded portion, and an ultrasonic wave emitted from an arc-shaped oscillating portion of the ultrasonic probe is applied to an outer surface of a welding pipe, The reflected wave from the welded portion is captured by the receiver, and the presence or absence of a defect in the welded portion is detected on the welding pipe production line based on the reception result.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. FIG. 1 is a perspective view schematically showing the entire configuration of a stepped defect detecting device for a welded pipe according to the present invention (hereinafter referred to as the present device).

In the figure, reference numeral 1 denotes a welding pipe. The welding pipe 1 is
A known pipe-making line in which a strip-shaped metal plate as a material is moved in the longitudinal direction, and during this movement, the both ends in the width direction are continuously welded while being bent into a cylindrical shape from both sides in the width direction. As shown in the drawing, the pipe is provided with a welded portion 2 formed in the axial direction and continuous at one position on the peripheral surface thereof.

The apparatus according to the present invention detects a step-like defect in the welded portion 2 formed by abutment welding on a pipe-making line, and after removing an excess portion formed inside and outside the welded tube 1, the welded portion 2 is detected. A pair of ultrasonic waves, which are targeted and are disposed opposite to each other outside the welded pipe 1 which moves in the direction shown by the white arrow in the drawing after the step of removing the excess metal part, so as to sandwich the welded part 2 from both sides. Probes 3 and 3 are provided.

FIG. 2 is an external perspective view of the ultrasonic probe 3. As shown in the drawing, the ultrasonic probe 3 used in the apparatus of the present invention is configured by attaching a vibrator 31 capable of oscillating and receiving ultrasonic waves to an inner peripheral surface of a support 30 having an arc shape. ing. The illustrated vibrator 31 is formed into a band shape having a constant width by a polymer piezoelectric element, and is attached to the inner peripheral surface of the support 30 over substantially the entire length thereof.

The polymer piezoelectric element is replaced with a ceramic vibrator such as zircon titanate (PZT),
Since it has been used in recent years, it can be easily formed into various shapes, and is rich in flexibility and has a large internal damping coefficient. The transducer 31 as shown in FIG. 2 can be configured by being bent and attached directly to the surface along the surface. Further, since the polymer piezoelectric element has a low dielectric constant, it is possible to form a large-area vibrator 31 as shown in the figure without worrying about an increase in electrical impedance. from here

FIG. 3 and FIG. 4 are front views showing an example of a desirable positional relationship of the ultrasonic probe 3 shown in FIG. In this figure, the ultrasonic probe 3 is a welding pipe 1
The welded portion 2 is arranged to face the welded portion 2 formed at one location of the peripheral surface of the vibrator 31 so as to be included within the central angle range of the vibrator 31. In the ultrasonic probe 3 arranged as described above, when a predetermined excitation voltage is applied to the vibrator 31, ultrasonic waves are oscillated in respective normal directions from respective parts of the vibrator 31, and the ultrasonic Propagates toward the focal point F while being converged within the central angle range of the vibrator 31, and reaches the outer surface of the welded pipe 1 including the welded portion 2.

As shown in FIG. 1, a support rod 4 extends along the movement path of the welding pipe 1 that moves with the welded portion 2 facing upward, and extends in the middle of the support rod 4. Is a U-shaped support leg so as to straddle the welding portion 2.
The ultrasonic probes 3, 3 configured as shown in FIG. 2 are fixed to the forked ends of the support legs 40 so as to obtain the arrangement shown in FIG. Have been.

The base of the support rod 4 is supported by a guide rail 41 installed in a direction perpendicular to the direction of movement of the welding pipe 1, and is provided with an actuator 42 using an electric motor or the like.
It is configured to be movable along the guide rail 41 in accordance with the operation of. An imaging device is provided at the tip of the support rod 4 with the movement path of the welding pipe 1 facing downward as an imaging area.
43 is installed.

The output of the image pickup device 43 is given to the position control unit 5 via the image processing unit 50, and the output of the position control unit 5 is given to the actuator 42. Image processing unit 50
Performs image processing of an input from the imaging device 43, extracts a position of the welding portion 2 within an imaging region, and provides a signal corresponding to this position to the position control portion 5. The position control unit 5 calculates a deviation between the current position of the welding unit 2 and a preset reference position, and issues an operation command to the actuator 42 to eliminate the deviation.

The position of the weld 2 continuously formed on the weld pipe 1 fluctuates due to torsion about the axis while the weld pipe 1 moves along the movement path. The operation of the position control unit 5 is such that the support rod 4 is moved in accordance with the fluctuation of the welding part 2 as described above, and the ultrasonic probes 3 fixedly supported by the support rod 4 2 to maintain the above-described predetermined positional relationship.

The ultrasonic probe 3 mounted as described above
3 is such that, as shown in FIGS. 3 and 4, the welded portion 2 is formed within the central angle range of the vibrator 31 on the inner surface of the welded portion 2 formed continuously in the longitudinal direction of the welded pipe 1. The ultrasonic wave oscillated by the vibrator 31 in this state at least partially reaches the welded portion 2 and is reflected by a step-like defect generated in the welded portion 2.

The propagation of the ultrasonic wave between the ultrasonic probes 3 and 3 and the welding pipe 1 is performed via a propagation medium interposed between the two. Water can be used as the propagation medium, and the intervening between the ultrasonic probes 3 and 3 and the welding pipe 1 is performed by placing the welding pipe 1 and the ultrasonic probes 3 and 3 in a water tank. This is realized by facing each other.

As described above, the reflected wave reflected from the step-like defect is generated in various directions according to the inclination of the step-like defect. However, since the vibrator 31 has an arc shape as described above, The reflected waves reflected in various directions can be reliably captured. FIG. 3 shows a case where the step-like defect occurring in the welded portion 2 has an end face substantially perpendicular to the outer surface of the welded pipe 1, and FIG. As shown by arrows in these figures, at least a part of the ultrasonic waves oscillated from the entire surface of the vibrator 31 is within the length range of the vibrator 31 as shown by arrows in these figures. And is caught in this return position.

Since the ultrasonic wave oscillated by the oscillator 31 is converged within the central angle range, even if the step-like defect has a slight height, the ultrasonic wave is surely reflected, and the oscillator 31 oscillates. It is possible to catch it. In the apparatus shown in FIG. 1, the output of the ultrasonic probe 3 capturing the reflected wave on the transducer 31 as described above is subjected to predetermined processing in each of the separate signal processing units 6 and 6, and is output to the recorder 7. And the recording is performed by the recorder 7. With this configuration, the welded portion 2 of the welded pipe 1
Can be easily determined based on the recorded contents of the recorder 7. In addition, it is also possible to make this determination automatically and feed back this determination result to the pipe production line of the welded pipe 1, for example, to use it for adjustment of a group of forming rolls. Can be eliminated online.

FIG. 5 is a front view showing another example of a desirable positional relationship of the ultrasonic probe 3 shown in FIG. In this figure, the ultrasonic probe 3 is
Not within the range of the central angle of, but within the range of the diagonal of the central angle,
That is, the welding portions 2 are arranged to face each other so that the welding portion 2 is included in an angle range in which the ultrasonic waves converging at the focal point F diffuse beyond the focal point F.

In the ultrasonic probe 3 arranged as described above, the ultrasonic waves emitted from each part of the vibrator 31 in the respective normal directions are converged within the central angle range of the vibrator 31 and focused. F, and reaches the welded portion 2 while diffusing and propagating beyond the focal point F, and is reflected by a step-like defect occurring in the welded portion 2. This reflected wave is generated in various directions according to the inclination of the step-like defect. However, since the vibrator 31 has an arc shape, it is reflected in various directions as shown by arrows in the figure. It is possible to reliably capture at least a part of the reflected wave, and it is possible to determine the presence or absence of a step-like defect based on a change in the output of the vibrator 31 generated in response to the reception.

FIG. 6 is a diagram showing a sound pressure distribution along the propagation path of the ultrasonic wave emitted from the ultrasonic probe 3 shown in FIG. As described above, the ultrasonic waves emitted from the respective portions of the arc-shaped vibrator 31 once converge on the focal point F and follow a propagation path that spreads after passing through the focal point F. The sound pressure of the ultrasonic waves In other words, in the range before reaching the focal point F, ie, before reaching the focal point F, the adjacent portions are affected by the reduction of the propagation width. Is shown. On the other hand, in the range after reaching the focal point F, the influence of the adjacent portions is eliminated by the expansion of the propagation width, and the distribution becomes uniform in the width direction as shown in the AB section in the figure. .

Therefore, when the arrangement shown in FIG. 5 is adopted,
Ultrasonic waves having a uniform sound pressure distribution reach the welded portion 2 and the reflected wave from the step-like defect has a stable intensity, and the output of the vibrator 31 generated in response to the reception of the reflected wave The presence or absence of the step-like defect can be determined with higher accuracy by the change.

FIG. 7 is an external perspective view showing another embodiment of the ultrasonic probe used in the apparatus of the present invention.
The ultrasonic probe 3 shown in the figure has a long first vibrator 32 and a short second vibrator 32 on the inner peripheral surface of an arc-shaped support 30.
33 are arranged side by side in the circumferential direction. The provision of the first and second vibrators 32 and 33 in this manner makes it possible to increase the sound pressure of the ultrasonic wave oscillated by the vibrator 33 having a small area and improve the detection performance. That's why.

As shown in FIG. 8 or FIG. 9, such an ultrasonic probe 3 is located within the central angle range of the first and second transducers 32 and 33 or within the diagonal range of the central angle. The welding portion 2 is used so as to face the outside of the welding pipe 1 so as to include the welding portion 2. At this time, it is desirable to position the second vibrator 33 having a higher sound pressure so as to face the welded portion 2 of the welded pipe 1. In order to realize such an arrangement, in FIG. In FIG. 9, the second vibrator 33 is positioned on the upper side.

FIG. 10 is a diagram showing a sound pressure distribution in the middle of the propagation path of the ultrasonic wave emitted from the ultrasonic probe 3 shown in FIG. Also in the ultrasonic probe 3, the sound pressure of the ultrasonic waves emitted from the first and second transducers 32 and 33 is within the range before reaching the focal point F as shown in the CD cross section in the figure. In the range after reaching the focal point F, a non-uniform distribution is shown in the width direction, and a uniform distribution is shown in the width direction as shown in the cross section AB in the figure. Further, as is apparent from this figure, the sound pressure of the ultrasonic wave emitted from the second oscillator 33 is generally higher than the sound pressure of the ultrasonic wave emitted from the first oscillator 32.

Therefore, when the ultrasonic probe 3 shown in FIG. 7 is used, the arrangement of the ultrasonic probe 3 with respect to the welding pipe 1 is as follows.
As shown in FIG. 9, more preferably, the welded portion 2 is included in a range of the diagonal of the central angle of the first and second vibrators 32 and 33, and more preferably, the central angle of the second vibrator 33 is By realizing the welding portion 2 to be included in the diagonal range, high detection performance can be obtained.

Finally, an embodiment in which the apparatus of the present invention is applied to an actual ERW pipe production line will be described. The specifications of this pipe production line are as follows. Tube making size: Outer diameter 38.1 mm to 114.3 mm, wall thickness 1.6 mm to 10.5 mm Pipe making speed: 100 m / min

As shown in FIG. 7, the used ultrasonic probe 3 has a long first vibrator 32 and a short second vibrator 33.
Are mounted side by side in the circumferential direction, and their specifications are as follows. Resonator size: 10 mm × 30 mm, 10 mm × 15 mm Focal length: 40 mm Resonant frequency: 10 MHz

Using such an ultrasonic probe 3, the welding part 2 of the welding pipe 1 moving in the pipe-making line is opposed in the water tank, and the image is taken by the imaging device 43 configured as shown in FIG. By the above-described operation of the position control unit 5 based on the result, the step-like defect was detected while moving the ultrasonic probe 3 following the welded portion 2.

Table 1 shows the S / N ratio (dB) of the output actually obtained by the ultrasonic probe 3 using a test welding tube in which step-like defects of various heights were artificially formed. The result of having investigated is shown.

[0043]

[Table 1]

As shown in this table, according to the apparatus of the present invention, an output having an S / N ratio of 16 dB can be obtained even for a slight step-like defect having a height of 0.05 mm. This S / N ratio exceeds the S / N ratio (= 12 dB), which is the lower limit of flaw detection, and according to the apparatus of the present invention, it is sufficient for a step-like defect having a height of about 0.05 mm. It became clear that detectability was obtained.

FIG. 11 is a view showing the result of performing defect detection in the actual pipe making process. As in FIG. 14, the horizontal axis represents the height of the step (mm), and the vertical axis represents the height. The S / N ratio (dB) of the received reflected echo is shown. According to the method of the present invention, as shown in the figure, even for a natural defect having a height of 0.1 mm or less, a reflected echo having an S / N ratio exceeding the lower limit (= 12 dB) is obtained in most of the natural defects. , FIG. 14
By comparing with the above, the advantage over the surface wave method is apparent.

[0046]

As described above in detail, according to the method and the apparatus of the present invention, an arc-shaped ultrasonic probe is provided outside the target welding pipe at a central angle or a diagonal of the central angle. The ultrasonic wave emitted from the ultrasonic probe is allowed to reach the outer surface of the welded pipe so that the welded portion is positioned within the range of the welded portion. Reflected wave from the step defect can be reliably received by the ultrasonic probe, the presence or absence of the step-like defect based on the reception result, with high accuracy regardless of the form, and the height of the step. It becomes possible to detect.

In the apparatus of the present invention, the method of the present invention is carried out while moving the ultrasonic probe to the welded portion of the welded pipe moving on the pipe manufacturing line. The presence of step-like defects can be detected with high accuracy, and the results can be fed back to the pipe-making line to prevent the occurrence of step-like defects before they occur. The present invention has excellent effects, for example, it is possible to stably produce a simple welded pipe.

[Brief description of the drawings]

FIG. 1 is a perspective view schematically showing the entire configuration of the apparatus of the present invention.

FIG. 2 is an external perspective view of an ultrasonic probe used in the apparatus of the present invention.

FIG. 3 is a front view showing an example of a desirable positional relationship of the ultrasonic probe shown in FIG. 2 with respect to a welded portion.

FIG. 4 is a front view showing an example of a desirable positional relationship of the ultrasonic probe shown in FIG. 2 with respect to a welded portion.

FIG. 5 is a front view showing another example of a desirable positional relationship of the ultrasonic probe shown in FIG. 2 with respect to a welded portion.

FIG. 6 is a diagram showing a sound pressure distribution in the middle of a propagation path of an ultrasonic wave emitted from the ultrasonic probe shown in FIG.

FIG. 7 is an external perspective view showing another embodiment of the ultrasonic probe used in the apparatus of the present invention.

8 is a front view showing an example of a desirable positional relationship of the ultrasonic probe shown in FIG. 7 with respect to a welded portion.

FIG. 9 is a front view showing another example of a desirable positional relationship of the ultrasonic probe shown in FIG. 7 with respect to a welded portion.

10 is a diagram showing a sound pressure distribution in the middle of a propagation path of an ultrasonic wave emitted from the ultrasonic probe shown in FIG.

FIG. 11 is a diagram showing a result of performing defect detection in an actual pipe making process using the apparatus of the present invention.

FIG. 12 is an explanatory diagram of a conventional step-like defect detection method.

FIG. 13 is an explanatory view of a conventional step-like defect detection method.

FIG. 14 is a diagram showing a result of performing a defect detection in an actual pipe making process by a surface wave method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Weld pipe 2 Weld part 3 Ultrasonic probe 4 Support rod 5 Position control part 30 Support body 31 Transducer 41 Guide rail 42 Actuator 43 Imaging device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshifumi Fukuda 1850 Minato, Wakayama-shi, Wakayama Prefecture S-term Control Engineering Co., Ltd. F-term (reference) 2F068 AA48 BB03 BB09 BB19 DD05 FF12 FF18 JJ22 KK13 KK17 KK18 LL15 NN02 2G047 AA07 AB01 AB08 BA03 BC07 CA01 GA03 GA06 GB11 GB18 GH06

Claims (3)

[Claims] 1. A band-shaped metal plate is bent from both sides in the width direction into a cylindrical shape, and the edges in the width direction are joined to the welded portion of a welded pipe. In the method of detecting a step-like defect caused by misalignment, an ultrasonic probe having an arc-shaped oscillating portion and a receiving portion is welded to a center angle of the ultrasonic probe or a diagonal range of the center angle. The reflected portion of the ultrasonic wave emitted from the oscillating portion from the welded portion is captured by the receiving portion, and the stepped defect based on the reception result is detected based on the reception result. A method for detecting a stepped defect in a welded pipe, characterized by determining the presence or absence of the welded pipe. 2. A stepped metal plate formed by bending a strip-shaped metal plate from both sides in the width direction into a cylindrical shape, and joining the edges in the width direction by abutment welding. In an apparatus for detecting a step-like defect caused by misalignment, an ultrasonic probe having an arc-shaped oscillating unit and a receiving unit, and the ultrasonic probe, the central angle, or the central angle of the Positioning means for positioning the outside of the welding pipe so that the welded portion is included in a diagonal range, and a result of receiving the reflected wave of the ultrasonic wave emitted from the oscillating portion from the welded portion at the receiving portion. Determining means for determining the presence / absence of the step-like defect on the basis of the step-like defect. 3. The pipe making method includes a step of bending a band-shaped metal plate into a cylindrical shape from both sides in a width direction and a step of abutting welding the edge in the width direction of the formed band-shaped metal plate. 3. The step according to claim 2, further comprising: means for detecting a welded portion of the welded pipe moving on the line; and position control means for changing a position of the ultrasonic probe based on a detection result of the means. Defect detector.